Editor’s note

We are very pleased to announce that Jenni McDonald has been selected to receive the Postdoctoral Excellence Award by the Ecological Society of America’s Plant Population Ecology section for her Journal of Ecology paper, “Transients drive the demographic dynamics of plant populations in variable environments.” This paper was part of a BES cross-journal Special Feature “Demography Beyond the Population” comprising 21 papers on this important topic. Congratulations Jenni!

David Gibson
Executive Editor, Journal of Ecology

Jenni McDonald is an early career researcher currently two years into her first postdoctoral research position at the University of Exeter. Her research applies comparative demographic approaches to explore the evolution of life histories. Using COMPADRE, a global database of plant demographic models, coupled with simulated life histories, she is exploring the demographic buffering hypothesis, a theory that predicts traits that have a large influence on fitness are stabilized against environmental change by buffering that trait. Her exploration of stochastic population dynamics in combination with the strong transient research background of colleagues (past and present) at the University of Exeter led to the research topic of the paper published in Journal of Ecology, which highlights the importance of transients in variable environments.

Transients in variable environments

The natural world is rarely constant. Consequently, studying the dynamics of wild plant populations in variable environments is of particular significance to ecologists seeking to understand life history evolution, and has major implications for management and conservation. 

Until recently, two major types of analysis have explored this environmental variability. First, stochastic analysis, which accounts for variation in vital rates with survival, growth and reproduction changing over time. Second, transient analysis has shown that a population’s response to changing environments depends on the population stage structure, i.e. the number of individuals occupying each (st)age class. For example, a greater number of reproductive individuals will result in accelerated population growth (boom), whereas a bias towards immature individuals will reduce expected population growth (bust). Ellis and Crone (2013) highlighted that these two key processes are not mutually exclusive. Both an individuals’ vital rates (survival, reproduction, growth, regression) and population responses to shifts in demographic structures ((st)age structures) contribute to stochastic population dynamics.

The contribution of differences in vital rates alone can be measured by the asymptotic rate of growth or decline of the population. This pivots on the assumption of equilibrium dynamics, where the asymptotic growth rate and associated stable stage structure is assumed to be constant over time. Transient indices account for departures away from a stable population structure in non-stationary environments and quantify the instantaneous boom/bust in each time-step.

Our study, published in Journal of Ecology, builds on this research to disentangle the absolute and net contribution of transients to the population dynamics of 277 plant populations, comprised of 132 species from the COMPADRE database. We tackle the question, “what is the relative contribution of transient boom and bust to the dynamics of plant populations in variable environments?”

Using a simulation approach, we decompose the dynamics of each time step in to asymptotics and transients, with our results highlighting that transients contribute to over half the dynamics in stochastic environments. This result was true for net dynamics as well as absolute dynamics, which accounts for the strength of opposing asymptotic and transient effects, emphasising the important role transients play in shaping the population trajectories of plants.

This analysis raises several potential hypotheses for further exploration. First, the number of life stages modelled influenced both the contribution of transients and asymptotics. Whether this is a consequence of life history complexity or an artefact of modelling design remains to be explored. We also found plant populations tended to boom in response to temporal changes. Is an ability to bounce back from disturbance an adaptive response to living in a variable world?

Our results emphasise the embedded nature of transients in stochastic population dynamics and reveal interesting evolutionary and methodological patterns. These findings, along with continued methodological advances in the study of transients, will prompt further comparative analysis of transient population dynamics and increase the practical utility of these methods for management and conservation.

Jenni McDonald
University of Exeter
United Kingdom